Series editors' preface - Wood Tools

138 Conservation of Furniture
poly(vinyl chloride). These may be encountered
in museum objects but are not suitable
for use as conservation materials as they either
have restricted solubility or are thermosetting.
In general, coatings are not complete vapour
barriers. They do not prevent the transmission
of vapours but reduce the amount transmitted
in a given time. This property is valuable in the
absence of stable environmental conditions.
The effect of coatings in reducing the rate of
dimensional change in hygroscopic materials,
especially wooden panels, has been well
demonstrated (Buck, 1961). Unfortunately,
many sensitive objects are coated only on one
side. When environmental conditions are stable,
water vapour will pass through most coatings
until equilibrium is established and in
most cases this will happen surprisingly
quickly. Some waxes, for example paraffin
wax, possess good barrier properties to water
vapour and are better in this respect than the
synthetic thermoplastic polymers used as conservation
coatings. The moisture exclusiveness
of finishes on wood is reviewed by Feist et al.
(1985).
4.3.4 Optical properties
The most important optical properties of coating
materials are clarity (transparency), gloss,
refractive index and colour. High clarity
requires that the refractive index is constant
throughout the sample in the viewing line. The
presence of interfaces between regions of different
refractive index causes scattering of light
and reduction in transparency. This can be
seen in otherwise transparent coatings containing
very fine air bubbles, or matting agents
such as wax or silica. Amorphous polymers
free from impurities are transparent unless
chemical groups are present that absorb visible
light. Crystalline polymers may or may not be
transparent. Where crystalline structures are
smaller than the wavelength of light then they
do not interfere with the passage of light and
the polymer is transparent. Where these structures
are greater in diameter than the wavelength
of light then light will be scattered,
providing the crystal structures have a different
refractive index (i.e. a different density) from
that of amorphous regions.
Transparency may be defined as the state
permitting perception of objects through or
beyond the material (coating). It can be
assessed as the fraction of normally incident
light that is transmitted with less than 0.1° deviation
from the direction of the primary beam.
Some coatings, although transparent, may have
a cloudy or milky appearance known as haze.
This can be measured as the amount of light
deviating by more than 2.5 from the transmitted
beam direction and is often the result of
surface imperfections.
When light falls on a material some is transmitted
through it, some is reflected and some
is absorbed. Transmittance is the ratio of light
passing through to the light incident and
reflectance is the ratio of reflected to incident
light. The gloss of a film is a function of the
reflectance and the surface characteristics of a
material. A perfect mirror-like surface, known
as a specular reflector, shows one extreme of
behaviour. At the other extreme, a perfect diffuse
reflector reflects light equally in all directions
at all angles of incidence. Several
different measures of gloss exist, each defining
a distinct aspect of appearance. These include
specular gloss, distinctness of image gloss, contrast
gloss and sheen. Specular gloss refers to
the reflection that occurs at the angle of reflection
equal to the angle of incidence of a beam
of light. Contrast gloss is the ratio of intensity
of light that is reflected at two different angles
relative to the surface. Distinctness of image
gloss refers to the distinctness of patterns of
light reflected from a surface. Sheen is a type
of reflection, possessed by velvet, where a matt
surface gives rise to pronounced specular
reflection at a small angle to the surface.
There is a relationship between viscosity and
gloss whereby, generally, it is easier to get a
high gloss with a polymer of low degree of
polymerization (and low molecular weight).
This is because the viscosity (resistance to relative
motion within the material) is lower at a
given solids concentration. Viscosity is also
affected by choice of solvent and by temperature.
Viscosity grade provides a convenient
index. High viscosity grade material becomes
resistant to flow at an earlier stage in drying.
Solvent coatings that form an immobile gel at
a point when they still contain appreciable solvent
tend to form a surface that follows the
irregularities of the underlying substrate. If a
coating dries with a rough surface its contrast
gloss and distinctness of image gloss will be